IMPORTANCE OF FLOW AND PARTICLE-SCALE HETEROGENEITY ON CO(II III)EDTAREACTIVE TRANSPORT/

The effect of flow and particle-scale heterogeneity on the applicabili ty of batch-derived reaction networks for Co(II/III)EDTA interaction w ith synthetic and natural Fe oxide-containing subsurface sediments was evaluated using batch and column systems. At pH 4.5 and 6.5, Co(II)ED TA reactions with sediments involved adsorption (within 0.2 to 2 h), f ollowed by iron dissolution, forming Co2+ and Fe(III)EDTA (within 10 t o 300 h) and oxidation, forming Co(III)EDTA (within 5 to 50 h). Reacti on parameters that were based upon numerous batch experiments, in some cases, poorly described reactive transport in columns. For five sedim ent/pH combinations, column breakthrough could be well described with systematic change in some reaction parameters which included: slower m etal-EDTA adsorption rates; a greater number of adsorption sites; slow er iron dissolution; and faster oxidation. Additional simulations were used to determine the significance and uncertainty in determining the se reaction parameters in the 15-reaction system with 22 species. Simu lations that incorporated particle-scale heterogeneity with spatial di stributions of adsorption site density, affinity, and rate could repro duce the column/batch trends that were observed, indicating that spati al averaging across reactive and nonreactive surface sites results in different single-valued reaction parameters needed to describe the col umn system as an equivalent porous media. The impact of solute advecti on and additional mass transfer in columns on accurately predicting co lumn behavior with batch-derived parameters was not responsible for th e batch to column disparity. This study demonstrates that while the na ture of the reaction network (i.e. reaction identity and stoichiometry ) is identical in batch and column systems, development of transport-r elevant reaction parameters is needed for nonlinear reactions as parti cle-scale heterogeneities can significantly influence the apparent man ifestation of the reaction network during advective flow. (C) 1998 Els evier Science B.V. All rights reserved.